Electrophotographic recording



Jan. 29, 1952 C, F CARLSON 2,583,546

ELECTROl-DHOTOGRAPHIC RECORDING Filed May 12, 1948 sas/wu ffm/va 71; 72

y fon/Mmm 0- Patented Jan. 29, 1952 UNIT-ED STATES PATENT OFFICEELECTBOPHOTOGRAPHIC RECORDING Chester F. Carlson, Woodside, N. Y.

Application May 12, 1948, Serial No. 26,663

8 Claims. l

This invention relates to methods and apparatus for the electrictransmission and recording of intelligence.

An object of the invention is to improve such methods and apparatus.

.Another object .is Ato provide methods and apparatus for convertingelectrical signals and the like into an electric charge pattern on anelectrophotographic plate and for converting an electric charge patternon an electrophotographic plate or other electrostatic record elementinto electric signals or variations.

A further object is to improve methods and de- -vices for facsimile,telepicture and phototelegraphic purposes, electric image recording andrelated purposes.

An additional object is to provide a method and apparatus for recordingand `reproducing sounds and related variations.

Other objects of the invention will be apparent from the followingAdescription and accompany- `ing drawings taken in connection with theappended claims.

The invention accordingly consists in the features of construction,combination of elements, and arrangement of parts as will be exemplifiedin the structures to be hereinafter described and the scope of theapplications of which will be indicated in the following claims.

In the accompanying drawings in which are shown by way of illustrationseveral possible embodiments ofmy invention:

Figure l1 iis a diagrammatic illustration partly in section. of atransmitting system and apparatus embodying features of the preesntinvention, and villustrating features of the method of the invention;

Figure 2 illustrates a receiving system and apparatus embodying otherfeatures of the invention and adapted to be associated with the systemof Figure 1, or for use in other systems;

Figure 3 shows a modiilcation of part of the transmitting apparatus;

Figure 4 is a diagram of a modified transmitting apparatus illustratingother aspects of the invention; and

Figure 5 is a detail view of a further modified transmitting apparatus.

In Carlson Patent 2,297,691 issued October 6, 1942 is described aprocess of Electrophotography and an electrophotographic plate for usetherein. The plate. in its preferred form comprises a conductivebacking, such as a sheet of metal. coated with a layer ofphotoconductive insulating material. An electrostatic charge pattern or2 latent image can be produced on kor in the layer by a suitablecombination of electric charging and photographic exposure, as describedin that patent.

'Carlson Patent 2,357,809 of September 12, 1944 shows in Figure l0 amodified form of electrophoto plate, and in Figure 11 a form of masterplate capable of acquiring an electrostatic latent image or chargepattern. Methods of producing charge patterns on such plates aredescribed. for example, on page 6 of that patent.

Another Carlson patent, namely 2,277,013 of March 17, 1942 describesmethods and devices for the Electric Recording and TransmissionofPictures and shows another form of electrophotographic element adaptedfor such purposes.

The present invention contemplates methods and apparatus for obtainingor transmitting electric signals or like variations from electrophoto-Agraphic plates of the kind described in Carlson Patents 2,297,691 and2,357,809. It is also contemplated that, in some instances, signals maybe transmitted from other electrostatic record elements such as masterplates of the kind shown in Figure 11 of Patent 2,357,809. Thisinvention further contemplates certain methods and apparatus forproducing an electric charge pattern or electrostatic latent image on anelectrophotographic plate from electric current signals or the like.

In a preferred embodiment the invention contemplates a scanning systemand method for detecting and transmitting element by element and line byline the electrostatic charge pattern on an electrophotographic plate orother electrostatic charge-carrying element, whereby pictures, in

telligence, sound recordings and the like may be transmitted overfacsimile systems or the like, or may be transferred from the plate orelement to another plate or element or electro-mechanically engraved inmetal, wax, plastic or other material to form printing cuts, soundrecordings and the like.

While a preferred embodiment of the invention is described herein, it iscontemplated that considerable variation may be made in the method ofprocedure and the construction of parts without departing from thespirit of the invention. In the following description and in the claims,parts will be identified by specific names for convenience, but they areintended to be as generic in their application to similar parts as theart will permit.

Referring to the drawings, Figure 1 shows the essential features of anapparatus for scanning esame an electrophotographic plate carrying acharge pattern or electrostatic latent image and for converting thecharge variations obtained by scanning in sequence individual areas ofthe plate into signal currents. The apparatus comprises a drum II),shown in cross-section mounted on a driven shaft II for rotation forscanning purposes. The electrophotographic plate I2 comprising a nexiblemetal foil or sheet I3 having a coating I4 of photoconductive insulatingmaterial is wrapped around drum I and secured at its ends in a V-shapedgroove I5 extending along the face of drum I0. The groove carries a rowof pins I6 along one of its faces for anchoring one end of the foil I3,which is suitably perforated to fit over the pins. The other end of thefoil is clamped rmly against the other side of the V groove by a clampbar I1, suitably arranged to be moved into and out of clamping position.Reference is made to Figures '1 and 8 of Carlson Patent 2,277,013 forfurther details of construction.

A carriage I8 Vis slidably supported on a pair of guides I9 for movementalong the side of the drum and a motor-driven advance screw extendingparallel to shaft I I meshes with a thread- `ed lug 2| on carriage I8 toadvance the carriage slowly along the side of the drum.

The carriage comprises a grounded metal frame 22 slidable in guides I9and integral upstanding wall 23 concavely rounded to follow the contourof the vcylindrical surface of drum Il) and thereby provide a concavewall uniformly spaced from the surface ofA the drum throughout its area.The spacing is sulicient to leave a narrow air gap between the surfaceof coating I4 of the electrophotographic plate and the concave surfaceof wall 23.

A small aperture 24 is formed in the middle of wall 23, the sides of theaperture being tapered or flared outward on the back face of the wall.'The Wall of aperture 24 which intersects vwith inner face 25 of wall 23is brought sharply to an angular edge defining the smallest diameter ofthe aperture. This diameter depends upon the width of the line it isdesired to scan on the plate I2. Thus, if it is desired to scan with aresolution of 100 lines per inch the diameter of aperture 24 ispreferably in the order of 0.01. This diameter may be somewhat greaterthan the width of the line to bescanned, however, and useful results areobtainable even with diameters twice that oi' the line width. Thespacing between face 25 and the surface of coating I4 is made as smallas possible and preferably is not greater than the line width, such as0.01 inch.

A scanning electrode 26 comprising a mounting screw portion 21 andpin-like electrode portion 28 having a flat or-slightly rounded end 32is mounted with end 32 disposed in the center of aperture 24 andsubstantially flushfwith inner face 25- of wall 23. The electrode issupported in an insulating bushing member 29 which is clamped on theback of wall 23 by a metal clamp ring 30 which is screwed on to the backof wall 23. Bushing 29 hasa metal sleeve insert 3I into which screw 21is threaded. The passage 33 throughbushing 29 is spaced from electrode28 to reduce the capacity between the electrode and grounded frame 22.

Shaft II carries a cam arm 34 at one end of drum I0. A cam-actuatedswitch 35 is mounted adjacent the end of the drum and comprises contactarm 36 and a cam-actuated movable spring arm 31, having its end 38 inthe path of travel of cam arm 34, so as to close contact 39 supported onspring 31 against contact 40 supported on arm 36 momentarily once duringeach revolution of drum IU. The end of cam arm 34 is positioneddiametrically opposite to clamp bar I1 and spring end 38 isdiametrically opposite to electrode 26 so that contacts 39, 40 areclosed momentarily at the instant clamp bar I1 passes the end 32 of theelectrode. Metal clamp bar I1 is continually grounded through shaft IIand the supporting framework so that as it passes electrode 26 it willnot induce any electric charge on the tip 32 of the electrode.

Switch 35 is connected in a Series circuit comprising a battery 4I andrelay 42 so that the relay is actuated each time contacts 33, 40 areclosed by cam 34. Relay 42, in operating, closes its front contacts 43to connect a source of electric potential to electrode 2B throughconductor 44. The source of potential may comprise a grid biasarrangement for a vacuum tube circuit, for example, battery 45 shown inFigure l, a positive terminal of which is grounded, and the negativeterminal Vof which is connected through a high resistance potentiometer46 to ground. The sliding contact 41 of the potentiometer is connectedthrough conductor 48 to the armature of relay 42 so as to be connectedthrough contact 43 to conductor 44 when the relay is actuated. A smallcondenser 49 may be connected between potentiometer contact 41 andground if desired.

A detector, amplifier and signal transmission circuit is connected toconductor 44 to receive signal variations from electrode 26 in anysuitable manner. Any one of several known circuits may be used. In thecircuit illustrated the grid terminal 5I of triode 50 is connected toconductor 44 and the output terminals 52, 53 extend to a suitableamplifier 54 in which the signal is amplied and modulated if desired,the resulting signal being used for any desired purpose or sent over asignal channel 55 which may comprise a wire circuit, a recording system,a radio transmission system, a sound transmission system or a telephonesystem.

The operation of the system shown may be as follows: Electrophotographicplate I2, carrying an electrostatic charge image on the surface ofcoating I4 is wrapped around drum I0 and clamped into position by clampbar I1. Carriage I8 is moved along its track until pin end 32 isadjacent to one end of drum I0 and the edge of plate I2. The drivemechanism is then started to rotate drum I0 and the plate I2 at auniform rate and to rotate screw 20 to advance carriage I8 slowly alongthe side of the drum so that end 32 of electrode 26 Will scan thesurface of coating I4 in a spiral path of the desired pitch. Thus, ifplate I2 carries a picture which it is desired to transmit over afacsimile system at lines per inch screw 20 will advance carriage I8uniformly at the rate of one inch for each 100 revolutions of drum I0.As cam 34 passes contact arm Y31 it colses contacts 39, 40 momentarilyat each revolution of the drum. This serves to operate relay 42 for aninstant to connect conductor 44 and grid terminal 5I to the grid biascircuit to apply a predetermined negative grid bias potential to thegrid and conductor. This potential is adjusted, prior to operation, bymoving potentiometer contact 41 to apply a negative bias to the grid oftriode 50 suflcient to prevent any substantial capture of electrons bythe grid. Hence, when contacts 43 are again opened, while grounded clampbar yI1 is still adjacent to electrode 26,'the grid potential willremain sub'- stantially constant.

As rotation of the drum carries the surface of electroph'otographicplate I2 past the end 32 of the electrode 26 this same potential ismaintained on the grid of the tube as long as the surface of plate I2which passes end 32 is not an electrically charged area, since the gridand connecting conductor 64 are insulated from any adjoining conductors.When an electrically charged elemental area of the coating I4 of plateI2 passes tip 32, however, a charge is induced on tip 32 by theproximity of the charged surface of the coating. Assuming that thecoating carries an electrostatic latent image in the form of a negativecharge, it will induce a positive charge on end 32 of the scanningelectrode. Since the grid and conductor 44 constitute an isolatedconductive body the positive charges induced on the tip 32 are drawn inpart from the grid, thereby increasing the negative bias of the grid andconse- ;quently reducing the plate current in the tube.

It will thus be apparent that as negatively charged elemental areascorresponding to parts of the electrostatic image pass the scanningelectrode the plate current of triode 50 will be decreased as long as acharged area is passing the electrode and that the plate current willagain return to normal due to return of positive charges from tip 32 tothe grid as soon as the charged area has passed by. The signal obtainedln the plate circuit will accordingly vary in accordance with thevariation in charge on the plate along the spiral scanned by theelectrode 26.

The signal, after amplification by amplifier 54, which may if desiredinclude means to modulate a standard carrier signal frequency with theplate current signal, is fed over a signal channel 55.

The nature of the signal channel will depend on the desired use of thesignal and may comprise a wire circuit leading to a facsimile receivingstation of conventional design or such as that shown in Figure 2,'or aradio broadcast channel,

or the circuit of a local recording device such as a facsimile recorder,or an electro-mechanical engraving device such as a printing plateengraver.

AIf the plate I2 carries an electrostatic record which it is desired toconvert to sound the signal channel may comprise a telephone or loudspeaker system, or an electro-mechanical engraving or embossing devicefor engraving or embossing a sound record on wax, metal, celluloseacetate or the like or an electro-magnetic recording device forrecording on wire or ferromagnetic discs. Other applications of thesignal will readily be apparent.

During scanning of plate I2 the grounded conductive wall 23 functions asa shield for electrode 26 so that it will be unaffected by charged areasof plate coating I4 which are not substantially directly under tip 32 ofthe electrode. The spacing between electrode tip 32 and the surface ofcoating I4 can be varied by screwing the electrode 26 in or out toobtain the desired relation between sensitivity of the scanning systemand the shielding effect of wall 23. During use it is necessary toprevent light from striking the charged coating I4 of theelectrophotographic plate by operating in a dark room or providing asuitable housing for the drum.

Once each revolution, when bar I1 passes the electrode, the grid bias isbrought to normal by the momentary closing of contacts 43 to correct forany slight drift in bias resulting from insula- B tion leakage orcapture of electrons by the grid.

The capacity between electrode 26 and ground is kept at a minimum andconductor 44 is made as short as possible to insure transmission of themaximum available signal to the grid. Since the charge on coating I4 isof relatively high density, an adequate Isignal is obtainable at thegrid.

The system will function similarly in scanning a plate carrying apositive charge image or both negatively and positively chargedelemental areas. When a positively charged area passes the end 32 thenegative grid bias will be decreased to allow more plate current toflow.

Instead of using electrophotographic plate I2 the system and method canalso be used for scanning and transmitting signals from a master plate,such as that shown in Figure 11 of my Patent 2,357,809, comprising asheet metal or other conductive backing carrying an insulating imagewhich is electrostatically charged. Plate I2 is simply replaced by themaster plate.

Other than as described and shown, the scanning and transmissionequipment may be of any well known type including the usualsynchronizing and framing arrangements where required. Various otherrefinements can be added but are not essential to an understanding ofthe present invention.

Referring to another aspect of my invention, Figure 2 illustrates afacsimile signal receiver which may, if desired, be used in conjunctionwith the transmitter of Figure 1 or independently to receive and recordother signals. It may also be used to record sound-modulated electricalvariations. The receiver may comprise an amplifier if required, such asamplifier 56, for amplifying and demodulating signals received oversignal channel 55, as shown, or from other sources. The demodulatedsignal is fed to a light beam modulator or light valve 51 of any knownform for modulating a beam of light from a light source such as lamp 58behind a diaphragm 61 provided with a light aperture. The modulatedlight beam is focussed by the lens system in tube 59 through a smallaperture 13 onto the surface of a coating 6I on an electrophotographicplate 60.

Plate 60 is mounted on a drum 62 for rotation on shaft 63 inside acylindrical metal housing 64 closely spaced to the surface of coating6I. The scanning member comprising light source, light valve, lens tubeand housing are all mounted on a sliding carriage 65 arranged to beadvanced parallel to shaft 63 by a motor driven advance screw 66 in thesame manner as the carriage of Figure l.

rlhe walls of housing 64 are turned outward along one side to provide apair of spaced outwardly-extending walls 68 forming an aperture or slotalong the side of the housing. A row of spaced needles 69 is mounted inthe slot by a supporting block 10 of insulating material, the needlesextending through the block toward drum 62 in spaced parallel relationand terminating in points spaced slightly from coating 6I of theelectrophotographic plate.

A 6000 volt D. C. source 1I, such as a transformer-rectifier system, hasits high voltage D. C. output terminal A connected by conductor 11 andswitch 18 to needles 69. The grounded terminal B is connected throughrheostat 16 to shaft 63 and hence to drum 62 and plate 60. A controlswitch 12 is provided in a supply conductor of the high voltage source.

In operation, coating 6I on eleotrophotographic plate 6-0 is first givena uniform electrostatic charge over its surface. This may be done byclosing switches 12 and 18 and rotating drum 62 a few times. A coronadischarge takes place from needles 69 to coating 6I thereby applying acharge to layer 6l. When switches 12 and 18 are again opened tode-energze the needles a uniform charge is retained over the surface ofcoating 6I.

It is also possible, of course to charge the plate rby frictionalmethods before it is placed on drum 62.

With the uniformly charged plate on drum 62 ready to use, carriage 65 ismoved to align aperture 13 with one end of the drum. Lamp 58 isenergized and the drive motor for shaft 83 and screw 66 is started torotate the drum and slowly move the aperture and associated lightprojecting system along the side of the drum. Any signals received overchannel 55 are amplified and demodulated by ampliiier-demodulator 56 andfed to light valve 51 which thereby varies the quantity of lightprojected from lamp 58 onto plate coating 6I, by varying the width orintensity of the light beam.

The controlled light beam, as it scans the photo-conductive insulatingcoating 6| in a spiral path, discharges a greater or lesser proportionof the charge held on the elemental areas scanned. Elemental areas whichreceive a greater quantity of light, due to transmission of more lightthrough the light valve, will be more fully discharged than areasreceiving less light. The result is an electrostatic image on platecoating 6I which corresponds to the signal variations received. When thelsignal is a facsimile picture signal, coming, for example, from thetransmitter of Figure 1, or from a conventional facsimile transmitter,and the rotations o drum 62 have been synchronized with the sending drum(such as drum I) an electrostatic latent image corresponding to theimage transmitted, will be formed on coating 6l. This can be used forretransmission or can be developed to render it visible in the mannerdescribed in my aforementioned Patents 2,291,691 and 2,357,809, forexample, by removing the plate and dusting the coating with anelectroscopic powder.

For best results it is preferred that adjacent widths of the pathtraversed by the light beam during scanning be contiguous or veryslightly overlapping in order to avoid leaving a fully charged stripbetween adjacent turns of the spiral scanned path.

A sound signal may be recorded in the same way and used subsequently toreproduce the sound in the apparatus of Figure 1 as long as the chargeremains on the plate. I have found that such plates can retain theircharge image for several days without difficulty if kept in the dark.The sound record can also be developed with powder and transferred andaffixed to paper or lrn surface. Recordings of other types of signalvariations can be made in 'the same ways.

It will be noted that light aperture 13 in Figure 2 comprises a smallhole in a metal cup 14 fitted into insulating sleeve so as to beinsulated from the frame. This cup is connected by conductor 19 to theslide of a potentiometer 80 which can be bridged across high voltageoutput terminals A and B by closing a switch 8|.

Another method of scanning may be carried out as follows: A potential isapplied to cup electrode 14 by closing switches 12 and 8l. Potentiometer88 is adjusted so that no corona discharge takes place from the bottomwall of the Cil cup to the coating. The plate is scanned with thecoating in uncharged condition. When an elemental area of the platepasses the aperture the light beam renders that area momentarilyconductive. The electric eld created by the cup electrode through thelayer attracts charges to the surface of the coating where they becometrapped by the return of the coating to insulating condition as itpasses out of the range of the light beam. The amount of charge thusbuilt up on an elemental area depends on the area of the spot of lightor the intensity of the light, according to the type of light valveused. Hence, the plate is charged in accordance with the signalvariations. The sign of the charge on the plate produced by this methodwill be the opposite to that obtained with the preceding method.However, either polarity of image can be obtained by reversing theconnections to terminals A and B.

According to a modiiied method oi scanning the charged plate, after auniform charge has been applied by needles 69, switch 18 is opened todisconnect the needles but switch 12 is left closed to maintain the highvoltage supply in operation. Switch 8l is then closed during scanning.This applies a high voltage to cup electrode 14 of the same sign as thecharge on coating 6 l. The potential on cup electrode 14 creates anintense field through layer 6I, in the vicinity of the aperture 13,reinforcing or accentuating the electrical eld tending to drive thecharge through coating 6I to the grounded backing of plate B8. Theeflciency of scanning is thereby improved.

By applying a small ch-arge density to the plate coating prior toscanning and creating an intense field by cup electrode 14 duringscanning it is possible to obtain an actual reversal of charge on thecoating in the areas traversed by the light beam when the light valvepermits the maximum light to reach the plate. There is thus built up animage of one polarity on a background of opposite polarity.

Figure 3 illustrates a modification of the transmitter -apparatus ofFigure 1. In this embodiment a scanning electrode 82 replaces electrode26. Electrode 82 is mounted for longitudinal vibration at ultrasonicfrequencies on a vibrator, such as piezo-electric crystal Vibrator 83.Piezo crystal 84 is supported rigidly at one end in a mounting 85secured to carriage I8 and electrode 82 is secured by mounting 88 to thefree end of the crystal to extend parallel to the direction ofvibration. A pair of electrodes 81 applied to the faces 0f the crystalare supplied with alternating current of a voltage and frequencysuflicient to vibrate the scanning tip 88 of the electrode toward andaway from the coating I4 of plate I2 carrying a charge image. Electrode82 is connected by conductor 89 to the grid of triode 58. highresistance 90 to the grid. During scanning the vibration of tip 88varies the capacity between the tip and the coating I4 at the frequencyof vibration so that an alternating voltage is superimposed on the gridbias whenever a. charged area of the coating passes the tip. Theamplitude of the variations will depend on the density of charge on theelemental area being scanned. The output of tube 50 will comprise analternating signal superimposed on the plate current. With thisarrangement the cam-actul ated contacts and relay are not used.

Grid bias battery 45 is connected through' scanning. lLight from lamp 9Ipassing through a slit in diaphragm 92 is focussed by the lens system intube 93 through a small aperture 94 in the wall of cylindrical metalhousing 95, which is spaced from the coating I4 of electrophotographicplate I2. The rotating assembly comprising shaft I I, drum II! and plateI 2 is insulated from housing 95. The spacing between coating I4 and theinside wall c! cylindrical housing 95 is kept as small as practicable,while still assuring clearance between the plate coating and thehousing, and may be in the order of 20 to 50 mils.

In the circuit of Figure 4 the grid of vacuum tube 59 is connected, byconductor 96, to metal housing 95. 'I'he rotating assembly is connected,by sliding contact 91 on shaft II and conductor 98, to the negativeterminal of grid bias battery 45, and through resistance 99 to the gridof tube 50.

It will be evident that the electrostatic latent image on coating I4will induce an opposite charge on the insider wall of cylindricalhousing 95. As soon as equilibrium is established, by current throughresistance 99, or by momentarily grounding the rotating assembly to thehousing, this static charge will have no effect on the grid potential.

In operation of Figure 4, the light beam system and cylinder 95 areslowly advanced along the side of drum I as the drum is rotated in themanner previously described. Plate I2, carrying an electrostatic latentimage on coating I4, is scanned in a spiral path by the light beam. Aseach elemental area of the coating is traversed by the light beam thecharge (or part of it) entrapped on that elemental area is released andflows through the coating to the metal backing layer I3. The smallquantity of charge which has been induced on housing 95 by the elementalarea Vof the plate is now released and flows through conductor 96,resistance 99 and conductor 98 to the metal backing I3 of the plate.During the brief interval of this current flow the potential on the gridof tube 50 is made less negative (if the current is positive) permittinga if momentary increase in tube output current and hence sendingr asignal impulse over the line. The strength of the signal during scanningis proportional to the charge on the plate area being scanned.

In some cases it may be desirable to superimpose a predeterminedfrequency on the signal by interposing a rotating toothed wheel lightchopper |00 in the light beam as shown in Figure 4.

Figure is a fragmentary View illustrating a modification of thetransmitter of Figure 1 or Figure 3. In this embodiment wall 23 isreplaced by a tubular metal scanning head |23 surrounding electrode pin28 and terminating in a reduced-diameter rim |25 around end 32 of thenin and spaced from the end. It is evident from this that the shieldneed not be a wall of eX- tensive area but can be simply a narrow wallor edge close to the end of the electrode, and the term wall as used inthe claims, is intended to include such a construction.

It is obvious that the mechanical arrangements may be varied in manyknown ways to facilitate the scanning operation. For example, the partshere illustrated as. mounted on a carriage for movement along the sideof the drum may be stationary and the drum itself may be advanced on ascrew to effectuate the scanning. Also, it is apparent that theelectrostatic record element can vary in form and that the signals maybe reproduced from, or recorded on, a ilat plate electrostatic recordelement or a tape electrostatic record element, such as a metal tapecoated with photoconductive insulating material in which electrostaticrecord comprises a single path or multiple paths lengthwise of the tape.

While the present invention, as to its objects and advantages, has beendescribed herein as carried out in specific embodiments thereof, it isnot desired to be limited thereby but it is intended to cover theinvention broadly within the spirit and scope of the appended claims.

What is claimed is:

l. In combination, a record member comprising an electrophotographicplate comprising a conductive backing and a coating of photoconductiveinsulating material thereon for carrying an electric charge; a scanningmember for scanning said surface, said scanning member comprising alight source and a projector for projecting a spot of light onto saidcoating from said source, a movable support for at least one of saidmembers to move said spot of light over the surface of said recordmember in a predetermined scanning path, and a light modulator formodulating said light beam in accordance with signal currents fed tosaid modulator.

2. The combination of claim 1, in which a corona discharge electrode ismounted in spaced relation to said coating and a high voltage source hasits output terminals arranged to be connected to said electrode and thebacking of said record element, said electrode and record element beingrelatively movable to bring said electrode successively over all partsof said coating to apply a uniform electrostatic charge to said coatingpreparatory to scanning.

3. In combination, a record element comprising an electrophotographicplate consisting of a conductive backing and a coating ofphotoconductive insulating material thereon for carrying an electriccharge image, a scanning element for scanning said surface, saidscanning element comprising a conductive wall uniformly spaced from saidcoating throughout its area and provided with a small aperture and alight source and light projector for projecting a spot of light fromsaid source through said aperture onto said coating, a movable supportfor at least one of said elements to move said spot of light over thesurface of said record element coating in a predetermined scanning path,and an amplifier, the backing of said element and said conductive wallbeing connected respectively to the input terminals of said amplier.

4. In combination, an electrophotographic plate comprising a conductivebacking and a layer of photoconductive insulating material overlyingsaid backing in contact therewith, charging means for applying anelectric charge uniformly over said layer, a light beam projector andmeans for effecting relative movement between said projector and saidlayer to scan said layer with said light beam, and a modulator forvarying the intensity of said light beam during scanning.

5. The combination as claimed in claim 4, in which means are providedfor selectively energizing said charging means or said light beamprojector.

6. In combination, an electrophotographic plate comprising a conductivebacking and a surface layer of photoconductive insulating materialoverlying said backing in contact therewith, a scanning member and meansto eiect relative movement between said plate and said member to scanthe surface of said plate with said member, said member comprising aconductive electrode overlying the surface of said plate and spacedtherefrom and having an aperture therein and a light lbeam projector forprojecting a light beam onto said plate through said aperture.

7. The combination as claimed in claim 6 in which a potential source isconnected between said conductive backing and said electrode.

8. In combination, a conductive cylinder coated with a surface layer ofphotoconductive insulating material, a scanning member comprising alight beam projector for projecting a spot of light onto said surfacelayer, and a modulator for modulating the intensity of said light beamin accordance with a signal, a charging electrode for applying anelectric charge to said surface layer, means for selectively energizingsaid scanning member and said charging electrode, means fory rotatingsaid cylinder with respect to said scanning member and chargingelectrode and means for moving said scanning member axially with respectto said cylinder.

CHESTER. F. CARLSON.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,897,796 Fulton Feb. 14, 19332,052,383 Cooley Aug. 25, 1936 2,143,214 Selenyi Jan. 10, 1939 2,197,050Kellogg Apr. 16, 1940 2,200,741 Gray :May 14, 1940 2,277,013 CarlsonMar. 17, 1942 2,283,148 Bruve May 12, 1942 2,297,398 Freis Sept. 27,1942 FOREIGN PATENTS Number Country Date 456,810 Great Britain Nov. 16,1936

